1. Field of the Invention
The present invention relates to the art of vehicle axles. More particularly, the invention relates to the art of raised-center axles for heavy-duty vehicles, such as tractor-trailers or semi-trailers and straight trucks, and processes for forming such axles.
2. Background Art
Heavy-duty vehicles, such as tractor-trailers or semi-trailers and straight trucks, typically include multiple axles that are longitudinally spaced along the vehicle to create ride stability. Each axle usually includes a central tube and a pair of spindles. The spindles are mounted on opposing ends of the central tube and extend outboardly therefrom. The wheels of the vehicle are rotatably mounted on the spindles, and a trailing or leading arm suspension system connects each axle to the vehicle frame. The suspension system and axle often are referred to in combination as an axle/suspension system. For the purpose of convenience, reference herein will be made only to axles, with the understanding that such axles are used in a heavy-duty vehicle axle/suspension system.
Certain types of heavy-duty vehicles, such as rear-discharge tanker trailers, often utilize an axle in which the central tube includes a center portion that is bent upwardly. Such axles are known in the art as raised-center axles, and the upwardly bent portion is referred to as a bend or a hump. When the raised-center axle is in an in-service position, the hump is above the rest of the axle or its unbent horizontally disposed portions. This is in contrast to a drop-center axle, in which the center tube of the axle includes a hump that is below the rest of the axle when the axle is in an in-service position. In the prior art, the processes associated with forming the hump for a raised-center axle have necessitated the use of a thick wall for the central tube, such as about 0.750 inches or greater.
For example, cold-forming processes of the prior art used to fabricate a raised-center axle involve a single-hit bending process, where the hit is performed by a hydraulic press. In such a process, an axle having a straight central tube is inserted into the press and a punch driven by the press hits a center portion of the tube, bending the center portion of the tube in one motion to form the hump. This single-hit process, however, produces residual stresses in the cross-section of the axle that are detrimental to the axle when it is loaded in service conditions. That is, since the hump of the raised-center axle is above the rest of the axle in an in-service position, the “overloading” of the central tube caused by the press in forming the hump produces compressive stresses in the bottom portion of the hump. However, after the punch retracts once the central tube has been bent to form the hump, there is a slight amount of spring-back of the central tube. This spring-back produces a state of residual tensile stress in the bottom portion of the hump of the axle, which combines with load forces experienced by the axle when it is in service that tend to flex the hump and to create additional tensile stresses in the bottom portion of the axle hump. This combination of load-induced tensile stresses and residual tensile stresses in the axle at the bottom portion of the hump has the potential to produce a premature failure of the axle.
The effect of the state of residual tensile stress in the bottom portion of the hump of the raised-center axle is in contrast to the effect of the same state of residual stress in a drop-center axle. More particularly, a drop-center axle includes the same state of residual tensile stress in the same location as the raised-center axle, however the inverted orientation of the drop-center axle causes in-service loading conditions to produce compressive stresses that counterbalance the residual tensile stresses. This contrast substantiates the principle that a one-time overload to form an axle tube produces residual stresses that are favorable to subsequent loading in the same direction, while those same residual stresses are detrimental to subsequent loading in the opposite direction.
To compensate for the residual stresses that contribute to the potential premature yielding of raised-center axles that are cold-formed according to prior-art processes, the axles include large wall thicknesses, on the order of about 0.750 inches. Even with such large wall thicknesses, however, these prior-art raised-center axles are still potentially susceptible to premature structural failure. Post forming treatment processes have been tried in an attempt to reduce such undesirable residual stresses in the prior art raised-center axles, such as by shot peening or needle peening of the axle. However, the addition of such post-forming treatment processes undesirably increases the time and expense to produce a raised-center axle. In addition, surface treatments such as those noted above do not significantly improve the static strength of the axle, which is diminished severely by residual stresses from cold-forming.
Other prior-art raised-center axle forming processes include hot forming of the hump. Hot forming reduces residual stresses, but requires a large wall thickness to maintain the ability to bend the axle central tube to form the hump while maintaining structural integrity. Thus, an axle tube wall thickness of about 0.750 inches is again required. Hot forming also is typically more expensive than cold forming, due in part to the additional time, energy and equipment associated with heating the axle for forming. As a result, raised-center axles that are hot formed tend to be more expensive than those that are cold formed, and still require increased wall thicknesses, which introduce unnecessary weight to the axle.
The large wall thicknesses needed for raised-center axles formed according to prior art processes are undesirable, as large wall thicknesses increase the amount and thus the cost of the raw material needed for the axle. In addition, a large wall thickness increases the weight of the axle, which undesirably contributes to decreased payload for the vehicle on which the axle is incorporated.
Thus, a need exists in the art for a raised-center axle that overcomes the problems of the prior art by being economically formed, while reducing undesirable residual stresses and wall thickness, and maintaining or improving physical properties exhibited by prior art raised-center axles. A need also exists in the art for a process to economically form such a raised-center axle. The present invention provides such a raised-center axle and method for forming the same.